The recovery of helium from helium-bearing natural gases by processes based upon the use of cryogenic techniques is known. Such processes basically consist of subjecting a helium-bearing natural gas to successively lower temperatures to condense and thereby remove from the natural gas those components therein having boiling points higher than that of helium. These components generally include, in descending order of their boiling points, hydrocarbons heavier than methane, methane itself and nitrogen.
In general, these known processes comprise at least three distinct operative steps or stages. These include (1) a preliminary gas treatment step for the removal of water, carbon dioxide and hydrogen sulfide, (2) a natural gas liquids separation step using low but noncryogenic temperatures and (3) a crude helium product separation step employing cryogenic temperatures, said product containing at least 50 volume percent of helium. When a pure helium product is desired a fourth step or stage will be integrated into the process to substantially reject nitrogen from the crude helium product. A general description of two known processes for cryogenically separating and recovering either crude or pure helium from helium-bearing natural gases is provided in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 10, pp 872-873, 2ed (1966). More detailed descriptions of the two processes described in the Kirk-Othmer reference are provided in the article "Helium Production At The Bureau Of Mines Keyes, Okla. Plant" by W. M. Deaton and R. D. Hayes, Bureau of Mines Information Circular, IC 8018 (1966) and in the article "A New Approach to Helium Recovery" appearing in Kellogram, pp 4-5, Issue No. 3 (M. W. Kellogg Co., 1963).
As is clear from a review of the above references, some of the required refrigeration employed in the processes is obtained through the use of expansion, i.e., the Joule-Thomson cycle, and indirect heat exchange. However, it also is clear from a review of these references that to attain the lowest temperatures (i.e., cryogenic temperatures) required in these processes, integrated but independent and separate liquefaction cycles (i.e., refrigeration) using e.g., methane or nitrogen as working fluids must be employed. The need for such auxiliary refrigeration contributes not only to an increase in the initial capital costs for helium extraction plants embodying these processes but also to an increase in both operating and maintenance costs for such facilities. Thus, a process for the separation and recovery of a crude helium product from a helium-bearing natural gas in which auxiliary refrigeration was not required to achieve the cryogenic temperatures necessary to the separation would represent an advancement over these known processes.